As one method of measuring the oxygen concentration in a target gas for measurement, there is known an absorption spectrometry that uses the fact that oxygen molecules absorb light of only a specific wavelength region (for example, 761 nm). Because this absorption spectrometry is able to measure the target gas without contacting it, it is possible not only to measure the oxygen concentration in the target gas for measurement without disturbance but also to measure it in an extremely short response time.
Among absorption spectrometric methods, in particular, “wavelength tunable semiconductor laser absorption spectroscopy” that uses a wavelength tunable semiconductor laser (laser element) as a light source, is realized by a simple device configuration. For example, with regard to a gas analyzer using the “tunable semiconductor laser absorption spectroscopy,” for a piping in which the target gas for measurement flows in a predetermined direction, it is common to add a wavelength tunable semiconductor laser and a light detection sensor (light receiving section) that are provided to face each other to form an optical path 1 (optical distance) across the piping through the incident optical window and the exiting window formed in the piping. (See, for example, Patent Document 1).
With such a gas analyzer, predetermined wavelength ν laser light (measurement light) is oscillated from the wavelength tunable semiconductor laser. The progress of the laser light is hindered by the shielding effect of the oxygen molecules present within the target gas for measurement in the course of passing through the pipe. Using the fact that the amount of light incident on the light detection sensor decreases in relation to the concentration of the oxygen molecules in the target gas, the concentration of the oxygen molecules is calculated by measuring the mount of the laser light I incident on the light detection sensor for the light amount of the laser light oscillated from the wavelength tunable semiconductor laser. FIG. 4 is a graph showing an example of an absorption spectrum obtained from the gas analyzer as described above. The vertical axis represents received light intensity I, and the horizontal axis represents wavelength ν. Note that I0(ν) (reference line) is the received light intensity where the absorption of the oxygen molecules is not received at wavelength ν and is derived by creating an approximate expression based on the received light intensity I of the non-absorptive wavelength.
From Lambert-Beer's law, the following equation (1) holds.
                              ln          ⁡                      (                                                            I                  0                                ⁡                                  (                  v                  )                                                            I                ⁡                                  (                  v                  )                                                      )                          =                  c          ×          l          ×                      S            ⁡                          (              T              )                                ×                      K            ⁡                          (              v              )                                                          (        1        )            
I0(ν) is the light intensity in the case of not receiving the absorption of oxygen molecules at wavelength ν, I(ν) is the transmitted light intensity at wavelength ν, c (mol/cm3) is the number density of oxygen molecules, l (cm) is the length (optical distance) of the optical path through the target gas for measurement, S(T) (cm−1/(mol/cm−2)) is a function of temperature T at a given absorption line intensity, and K(ν) is the absorption profile function.
FIG. 5 shows a schematic diagram of an example of a gas analyzer using wavelength tunable semiconductor laser absorption spectroscopy. Note that the direction horizontal to the ground is defined as X direction, the direction perpendicular to the X direction that is horizontal to the ground is the Y direction, and the direction perpendicular to both the X direction and the Y direction is the Z direction.
A gas analyzer 101 includes a light source unit 10, a light receiving unit 20, a gas temperature sensor (not shown), and a control unit 140 constituted by a microcomputer or a PC.
The gas analyzer 101 is provided to measure an oxygen concentration Cn within a measurement target gas flowing in a sample flow passage 70 that connects to each line of supply and exhaust of the fuel cell system. The sample flow path 70 extends in the Z direction, and on the side wall of the sample flow path 70, there are formed a lens 35 serving as an incident optical window and a lens 36 serving as an exit optical window disposed so as to face the lens 35 with a distance 1 apart in the −X direction. The measurement target gas flows in the sample flow path 70 in the Z direction.
The light source unit 10 has a semiconductor laser 11 (for example, a distributed feedback system for optical communication (DFB: distributed feedback) semiconductor laser diode and so forth), a lens 13, and a D/A converter 12. Further, the laser light from the semiconductor laser 11 is configured to pass through an optical fiber 33 and a lens 13 in the −X direction and travel from the lens 35 into the sample flow channel 70 to irradiate the measurement target gas provided in the sample flow path 70.
Furthermore, the light source unit 10 of this type converts a drive current value for application to the semiconductor laser 11 by a predetermined cycle n; that is, specifically, by applying the drive current value of a saw-tooth shape, a laser beam having a predetermined wavelength range (sweep width) ν1 to ν2 is oscillated from the semiconductor laser 11 at a predetermined period n. FIGS. 6a and 6b represent conceptual diagrams showing a relationship between the drive current value and the oscillation wavelength ν of the laser light; that is, FIG. 6(a) is a waveform diagram of the drive current value for application to the semiconductor laser 11; and FIG. 6(b) is a wavelength diagram of the oscillation wavelength ν of the laser light oscillated from the semiconductor laser 11 to which the drive current value is applied.
The light receiving unit 20 may be any device as long as it can convert the light intensity I into an electric signal; for example, a photodiode 21 may be used. The photodiode 21 then is positioned to receive the laser light emitted in the −X direction outside the path 70 from the lens 36 via an optical fiber 34 and a lens 23 and receives the intensity I of the laser beam that has passed through the measurement target gas.
It is known that interference noise (fringe noise) is produced when different laser light traversing the optical distance 1 is reflected multiple times by the lenses 35, 36 and so forth and received by the photodiode 21. This interference noise cannot be completely eliminated even if low reflective materials are used for the lenses 35, 36, and so forth.
Therefore, by measuring only the interference noise with the photodiode 21 or extracting noise from the intensity I received from the photodiode 21 before measuring the oxygen concentration Cn and by subtracting in advance the measured or extracted interference noise from the received intensity I when measuring the oxygen concentration Cn, a corrected light intensity I from which the interference noise is removed can be created. Alternatively, by creating a light intensity as changed by time Iν(t) at a wavelength ν, and by performing a fitting process using a signal (quadratic function) that matches the physical phenomenon for the time-changing Iν(t), a corrected light intensity, time-changing iν(t), from which the interference noise is removed can be made.
In each period n, the control unit 140 reads from the photodiode 21 through the A/D converter 22 the intensity In(ν1) to In(ν2) of the laser light and creates the corrected light intensity in(ν1) to in(ν2), and therefore, the oxygen concentration Cn can be calculated based on equation (1).
On the other hand, in the present automobile industry, there is a great demand to measure the hydrocarbon concentration and the like in exhaust gas from an internal combustion engine and the like. Therefore, it is conceivable that a gas analyzer 101 could be used to measure the density (specific gas amount information) Cn of oxygen molecules (specific gas) in the measurement target gas within a combustion chamber of an internal combustion engine.